A Rocking-chair Rechargeable Seawater Battery

Seawater batteries are attracting continuous attention because seawater as an electrolyte is inexhaustible, eco-friendly, and free of charge. However, the rechargeable seawater batteries developed nowadays show poor reversibility and short cycle life, due to the very limited electrode materials and complicated yet inappropriate working mechanism. Here, we propose a rechargeable seawater battery that works through a rocking-chair mechanism encountered in commercial lithium ion batteries, enabled by intercalation-type inorganic electrode materials of open-framework-type cathode and Na-ion conducting membrane-type anode. The rechargeable seawater battery achieves a high specific energy of 80.0 Wh/kg at 1,226.9 W/kg and a high specific power of 7,495.0 W/kg at 23.7 Wh/kg. Additionally, it exhibits excellent cycling stability, retaining 66.3% of its capacity over 1,000 cycles. This work represents a promising avenue for developing sustainable aqueous batteries with low costs.


Introduction
The variability of renewable energy sources like solar, wind, and geothermal necessitates dependable electrochemical energy storage systems for large-scale grid storage [1][2][3][4][5][6].For grid-scale stationary applications, affordability and safety are crucial [7,8].Among a variety of rechargeable batteries, aqueous sodium ion batteries are considered promising options for large-scale energy storage, owing to their affordability, nontoxicity, inherent safety, and high abundance [9][10][11][12][13], which utilize Na 2 SO 4 , NaCl, NaNO 3 , or NaClO 4 aqueous solutions as electrolytes.The seawater electrolyte can effectively address the sustainable challenges in rechargeable batteries requiring harmless and earth-abundant electrolytes and electrodes [14], because of its merits of being inexhaustible, eco-friendly, and free of charge.
The mature seawater batteries early developed are primary batteries [15][16][17], where Mg or Al metals serve as anodes, and AgCl or dissolved oxygen serves as cathodes.Rechargeable half-seawater batteries have been proposed recently [18][19][20][21][22], which use air cathodes working in seawater; however, the anodes (typically Na metal or hard carbon) can only work in conventional nonaqueous electrolytes, making it combustible and expensive.It is noted that supercapacitors can directly use seawater as the only electrolyte [23][24][25], but their specific energies are too low.For example, the specific energy of the supercapacitor based on an activated carbon is only 7.7 Wh/kg [23].Rechargeable full-seawater batteries (RSWBs), which employ seawater as both catholyte and anolyte, are rarely reported due to the limited electrodes.We recently developed a lattice-expansion strategy to enable anatase TiO 2 as a high-capacity and high-rate anode for RSWBs [26].High specific energies can be obtained in this RSWB; however, the energy efficiency is low and the cycling stability is still unsatisfactory, because an air cathode is required [26].It is thus urgent yet challenging to explore new low-cost electrode materials working with more feasible modes.
Compared to conventional aqueous sodium ion batteries, the intercalation of Na + may be interfered or hindered by other ions in RSWBs because of complex compositions of natural seawater.Among electrode options for RSWBs, it is expected that the electrode materials should meet one of the following 2 criteria: (a) they could intercalate multiple ions; (b) they can intercalate a kind of ion from seawater and simultaneously exhibit strong anti-interference ability for other ions.In this regard, open-framework crystal structures with large interval space, like Prussian blue analogs (PBAs), can accommodate multiple ions with a good structural deformation tolerance [27][28][29].We also note that, in rechargeable half-seawater batteries, a Na-ion conducting membrane (NASICON) is employed to separate the seawater catholyte and the nonaqueous electrolytes, while allowing only Na + transport between the 2 electrolytes [18][19][20][21][22].The good stability of NASICON-type materials in seawater hints their strong ion anti-interference ability and mechanical robustness, which makes them possible electrode materials for RSWBs.
The battery operates on the basis of a rocking-chair mechanism by using intercalation-type inorganic electrode materials, which is just the same as commercialized lithium-ion batteries work.The RSWB exhibits a high specific energy of 80.0 Wh/kg at 1,226.9 W/kg and a high specific power of 7,495.0W/kg at 23.7 Wh/kg.Additionally, it exhibits excellent cycling stability, retaining 66.3% of its capacity over 1,000 cycles.This work may promote the development of sustainable and low-cost aqueous batteries.

Results and Discussion
Open-framework-type cathode Numerous PBA cathodes have been applied in aqueous batteries, yet none of them demonstrate high capacities in RSWBs.It is reported that Mn-rich or Co-rich PBAs show relatively high capacities in aqueous sodium or potassium ion batteries [29,30].In this investigation, Co-rich PBA was utilized as cathode for RSWBs, and Mn-substitution strategy was further applied to improve its capacities.The cathode material was synthesized at room temperature using a coprecipitation method, employing low-cost raw materials of K 3 Fe(CN) 6 , cobalt chloride, manganese chloride, and sodium citrate.All of the Bragg diffraction peaks in the x-ray diffraction (XRD) pattern of the optimized sample, with a Co/Mn atomic ratio of 0.8/0.2 and the addition of sodium citrate, can be well indexed to orthogonal phase PBA (Fig. 1B) [31].The cell constants a, b, and c and cell volume V were determined to be 10.10, 7.14, and 7.14 Å and 514.58 Å 3 by Rietveld refinement with a robust fit.Figure 1C displays the Fourier transform infrared spectrum (FTIR), which clearly shows the characteristic absorption peaks corresponding to the Fe-C≡N, H-O-H, C≡N, and O-H vibrations [32], further verifying the successful synthesis of the PBA.The atomic ratio of K/Co/Mn/Fe, determined by inductively coupled plasma optical emission spectrometry, is found to be 0.97/0.8/0.2/0.81.The thermogravimetric curve (Fig. S1), indicates a weight loss of 12.9%, corresponding to 2.2 H 2 O in this PBA.Hence, the chemical formula of the prepared sample can be expressed as K 0.97 Co 0.8 Mn 0.2 [Fe(CN) 6 ] 0.81 •2.2H 2 O.
To evaluate the electrochemical performance of the K 0.97 Co 0.8 Mn 0. The electrochemical performance of the K 0.97 Co 0.8 Mn 0.2 [Fe (CN) 6 ] 0.81 •2.2H 2 O was further evaluated by galvanostatic charge-discharge (GCD) (Fig. 3C).Consistent with the above CV results, 3 discharge plateaus are observed.At the current densities of 0.2, 0.5, 1, 2, 3, and 5 A/g, the specific capacities of K 0.97 Co 0.8 Mn 0.2 [Fe(CN) 6 ] 0.81 •2.2H 2 O are 105.2,94.5, 88.6, 82.7, 77.4, and 62.4 mAh/g, respectively (Fig. 3C and D).The discharge capacity recovers to 101.78 mAh/g when the current density was reduced to 0.2 A/g, indicating excellent rate capacity and high reversibility.To understand the kinetic properties as a function of the charge/discharge depth, the galvanostatic intermittent titration technique was subsequently utilized (Fig. 3E).The diffusion coefficient is from 10 −11 to 10 −9 cm 2 /s in the charge process, while it maintains a high level (~10 −9 cm 2 /s) during the discharge process, conducive to the outstanding rate capability.Additionally, the K 0.97 Co 0.8 Mn 0.2 [Fe(CN) 6 ] 0.81 •2.2H 2 O shows attractive cycling stability, with 87.6% capacity retention over 1,000 cycles at 3 A/g (Fig. 3F).The initial gradual rise in capacity can be ascribed to the activation process, which occurs as the electrolyte gradually infiltrates the porous electrode [37].
The capacity of the Mn-substituted Co-based PBA surpasses those of both the Mn-based PBA and Co-based PBA (Fig. 3G).The optimized atomic ratio for Co/Mn was 0.8/0.2 for simultaneously achieving high capacity and good cycling stability (Fig. 3 and Fig. S5).Although the crystalline water contents in the Mn-based (27.4%),Co-based (29.4%), and Co 0.8 Mn 0.2based (27.6%)PBAs are close (Fig. S6), the morphology and size are quite different (Fig. S7).The Mn-based PBA exhibits a regular cube morphology, while its size is in micrometer scale, which leads to the low specific capacity.On the contrary, although the Co-based PBA possesses nanoscale particle size, its appearance is irregular.The Co 0.8 Mn 0.2 -based PBA combines the advantages of small particle size and high crystal quality with a regular shape, contributing to the higher specific capacity.The observed capacity decay in Mn-based PBA may originate from the structural deformation of Mn-N 6 octahedra due to Jahn-Teller distortion during Na + /K + ion insertion/extraction, which leads to the dissolution of active materials in the electrolyte [38].Introducing cobalt into Mn-based PBAs is effective in alleviating Jahn-Teller distortion and maintaining the stability of the crystal structure [39,40], thereby extending the cycle life.Furthermore, the sodium citrate utilized in our synthesis process also contributes to the improvement in the cycling stability (Fig. 3F and Fig. S5), owing to its capability for increasing crystal quality [41].It was found that the presence of sodium citrate during the preparation process can also increase the potassium content and reduce the crystal water content in the obtained PBAs (K 0.97 Co 0.8 Mn 0.2 [Fe(CN) 6 ] 0.81 •2.2 H 2 O versus K 0.08 Co 0.8 Mn 0.2 [Fe(CN) 6 ] 0.7 •4.45H 2 O) due to its reducibility.Therefore, the doping strategy and the crystal quality regulation developed herein contribute to the enhanced specific capacity and cycling stability.
The optimized PBA at fully charged and discharged states were further characterized by XRD, XPS, and EDX for comparative analysis.The peak near 27° in the XRD patterns (Fig. 4A) originates from the carbon cloth current collector and remains unchanged throughout the charge/discharge process.Compared with the discharged state, the Bragg diffraction peaks of sample at charged state shift toward higher angles.This lattice contraction when charged from 0.05 to 1.1 V is ascribed to the cation deintercalation reaction.After the first fully discharged (cation intercalation) process, the XPS peak intensities assigned to Na + and K + obviously increase compared to the first fully charged state (Fig. 4B and C).Subsequently, an increase in Na + and a decrease in K + can be found in the second discharge state (Fig. 4B to D).The Na/K ratio in discharge state increase from 0.78 in the 1st cycle to 2.4 in the 500th cycle (Table S1), indicating that the main charge carrier is Na + .The electrochemical properties in different aqueous electrolytes were also compared to further analyze the types of key charge carriers.The capacity follows the order of Na + >K + >Mg 2+ >Ca 2+ (86.6, 58.5, 25.2, and 22.8 mAh/g, respectively), as shown in Fig. S8.Although the leading charge carrier is Na + , we used K 3 Fe(CN) 6 instead of Na 3 Fe(CN) 6 in the cathode preparation, owing to the much lower price of the former.

NASICON-type anode
NASICON-type NaTi 2 (PO 4 ) 3 consists of an open 3D framework of TiO 6 octahedra and PO 4 tetrahedra, which may have potential applications in RSWBs.Hydrothermal process was used to synthesize NaTi 2 (PO 4 ) 3 , which was subsequently coated with a carbon layer to enhance both its conductivity and stability.As confirmed by the XRD pattern (Fig. S9A), well-crystallized NASICON-type NaTi 2 (PO 4 ) 3 was successfully prepared.The FTIR spectrum distinctly exhibits characteristic absorption peaks corresponding to P-O and Ti-O bond vibrations (Fig. S9B) [42,43].Raman spectroscopy further confirms the presence of Ti-O and P-O bonds and carbon coating (Fig. S9C) [43].The weight percentage of the carbon coating in the NaTi 2 (PO 4 ) 3 /C is approximately 1.7 wt.%, quantified from thermogravimetric analysis in air (Fig. S9D).As shown in Fig. S10A  and B, the NaTi 2 (PO 4 ) 3 /C demonstrates a polyhedron shape with the size of hundreds of nanometers.The parallel lattice fringes in the HRTEM image (Fig. S10C) are assigned to the (113) crystallographic planes of NaTi 2 (PO 4 ) 3 .As shown in Fig. S10C, the carbon layer on the surface of the NaTi 2 (PO 4 ) 3 is ca.4.3 nm in thickness.Additionally, the compositions are uniform in the as-prepared NaTi 2 (PO 4 ) 3 /C (Fig. S10D).The average pore diameter is 28.7 nm, and the specific surface area is 13.2 m 2 /g (Fig. S11).The chemical compositions of anode materials were analyzed by XPS (Fig. S12).The existence of Ti, P, Na, O, and C elements was revealed by the survey XPS spectrum of anode materials.The Ti 2p spectrum contains 2 peaks, corresponding to the characteristic Ti 4+ 2p 3/2 (460.7 eV) and Ti 4+ 2p 1/2 (466.4eV) [44].The O 1s spectrum can be deconvoluted into 3 components corresponding to the P-O-H (531.9 eV), O-C (532.7 eV), and P-O-Na or P-O-Ti or P=O (531.4 eV) [45].The fitted C 1s spectrum shows 3 main peaks at 284.8, 286.3, and 288.6 eV, which correspond to C-C/C=C, C-O, and O-C=O, respectively [46,47].
CV measurements (Fig. 5A) were performed at different sweep rates spanning from 0.2 to 1.0 mV/s.A pair of sharp peaks can be clearly observed in the CV curves.The CV curve shapes stay consistent across different scan rates, with only a slight shift in the potentials of the redox peaks.The b values of oxidation and reduction peaks are 0.63 and 0.54, respectively, slightly higher than 0.5, which indicate that Na + storage in NaTi 2 (PO 4 ) 3 /C is predominantly diffusion-controlled intercalation.Consistent with the CV results, clear charge/ discharge plateaus corresponding to the Na + intercalation/ deintercalation can be observed (Fig. 5B).The specific capacities of NaTi 2 (PO 4 ) 3 /C are 97.3,96.7, 93.5, and 72.9 mAh/g at 1.1, 1.3, 2.7, and 6.7 A/g, respectively (Fig. 5C).The specific capacity recovers to 90.6 mAh/g when the current density was reduced to 1.1 A/g, indicating the excellent rate capacity and high reversibility.The fluctuation of coulombic efficiency in Fig. 5C may be attributed to the side reactions of the anode material, such as hydrogen evolution reactions (Fig. S13).At lower current densities, the more severe hydrogen evolution reaction results in the lower coulombic efficiency.The cycling stability evaluation of the NaTi 2 (PO 4 ) 3 /C at 2.7 A/g reveals a high capacity retention of 65.6% after 1,000 cycles (Fig. 5D).
Upon comparing the GCD profiles in different electrolytes with identical cation concentrations, it was found that Na + is the key charge carrier for the energy storage of the NaTi 2 (PO 4 ) 3 /C (Fig. S14).The specific capacities in 0.5 M NaCl, KCl, MgCl 2 , and CaCl 2 are 73.8,8.3, 6.8, and 6.6 mAh/g, respectively.Ex situ XRD and XPS results were obtained at fully charged and discharged states of the NaTi 2 (PO 4 ) 3 /C to further elucidate the Na + storage mechanism.No obvious peak changes occur in the charge/discharge process, as illustrated in Fig. S15.This observation indicates that the Na + intercalation/deintercalation process occurs without phase transformation, contributing to the good structural stability.After the first discharge (Na + insertion) process, the intensity for the Na 1s peak in the XPS spectrum sharply increases compared to that in the fully charged state (Fig. 5E and Fig. S16).In addition, the EDX analysis for pristine, fully discharged, and fully charged samples reveals the changes in the content of Na + in the anode throughout the charge/ discharge processes, thereby supporting the inference that the energy storage is primarily governed by the Na + intercalation/ deintercalation process (Table S2).

Full battery
In our full cell design, the specific capacity of the K 0.97 Co 0.8 Mn 0.2 [Fe(CN) 6 ] 0.81 •2.2H 2 O is close to that of the NaTi 2 (PO 4 ) 3 /C (Figs.3D and 5C), which is beneficial for achieving high specific capacities for a full battery.The mass ratio of the NaTi 2 (PO 4 ) 3 /C and K 0.97 Co 0.8 Mn 0.2 [Fe(CN) 6 ] 0.81 •2.2H 2 O is set as 1.1:1, and natural seawater was used as electrolyte.A full battery was operated in 0 to 2.3 V. Three pairs of oxidation/reduction peaks in the CV curves are observed (Fig. 6A), which is similar to the cathode (Fig. 3A).At be also ascribed to the side reactions of the electrode materials.The full battery exhibits excellent cycling stability with 66.3% capacity retention over 1,000 cycles at 5 A/g (Fig. 6D).The cathode material demonstrates a relatively higher specific capacity in 0.5 M KCl aqueous electrolyte (Fig. S8B), while the anode material exhibits a very low specific capacity in this electrolyte (Fig. S14B and Table S2).Based on the rocking chair mechanism and the very low concentration of K + in seawater (the Na/K atomic ratio in the seawater is 39/1), the main carrier in the full cell is Na + instead of K + .
The specific energy and specific power of the full battery were calculated based on the mass of the active materials of both the NaTi 2 (PO 4 ) 3 /C and K 0.97 Co 0.8 Mn 0.2 [Fe(CN) 6 ] 0.81 •2.2H 2 O.The full cell delivers a high specific energy of 80 Wh/kg at a high specific power of 1,226.9W/kg, which are superior to the most previous energy storage devices that use seawater as the only electrolyte (Fig. 6E) [23,[48][49][50][51][52][53].A maximum specific power can achieve 7,495 W/kg.Furthermore, the energy efficiency is 63.3% at 5 A/g, much higher than 24.0% at 3.4 A/g for the full seawater battery that we recently reported [26].The specific energy is even comparable to the state-of-art aqueous Na + batteries [10,[54][55][56].As a proof of concept to exhibit the potential applications, a pouch cell assembled by the K 0.97 Co 0.8 Mn 0.2 [Fe(CN) 6 ] 0.81 •2.2H 2 O cathode and NaTi 2 (PO 4 ) 3 /C anode was constructed.The pouch cell (5 cm × 5 cm in size) can illuminate the light-emitting diode display screen (with a rated voltage of 1 V and a rated power of 0.056 W) for about 4 min (Fig. 6F).

Conclusion
In summary, we propose a rechargeable seawater battery that operates on a rocking-chair mechanism by using intercalation-type inorganic electrode materials.The cathode and anode materials are open-framework-type PBAs and NASICON-type NaTi 2 (PO 4 ) 3 , respectively.The constructed rechargeable seawater battery can achieve simultaneously high specific energy, high specific power, greatly enhanced reversibility, and long cycle life, as well as greatly enhanced energy efficiency.The work may open new perspectives for the development of high-performance sustainable rechargeable aqueous batteries.

Materials and Methods
The materials and methods can be found in the Supplementary Materials.
2 [Fe(CN) 6 ] 0.81 •2.2H 2 O in natural seawater, electrochemical tests were carried out in a standard 3-electrode system.The ion concentrations of Na, K, Ca, and Mg in natural seawater were determined to be 10,147, 440, 407, and 120 mg/l by inductively coupled plasma optical emission spectrometry, respectively.According to the cyclic voltammogram (CV) curves depicted in Fig. 3A, 3 pairs of redox peaks can be observed, suggesting 3 active sites for the energy storage in the K 0.97 Co 0.8 Mn 0.2 [Fe(CN) 6 ] 0.81 •2.2H 2 O.The relationship of the current (i) and the scan rate (v) follows a power law of i = av b .The b value equal to 1.0 represents surface-controlled electrochemical reaction, while b = 0.5 means that the electrochemical reaction is controlled by semi-infinite diffusion.The b values of A1 to A6 are 0.878, 0.798, 0.739, 0.92, 0.78, and 0.877, respectively (Fig. 3B).The high b values (0.73 to 0.92) suggest a high pseudocapacitive contribution, because the open-framework crystal structure allows fast ion transport.

Fig. 3 .
Fig. 3. Electrochemical performance of cathode material.(A) CV profiles at different scan rates, (B) the corresponding log(i)-log(v) curves for the peaks from A1 to A6, (C) GCD curves at different current densities, (D) rate performance, (E) galvanostatic intermittent titration technique response and ion diffusion coefficients, and (F) cycling stability at 3 A/g for K 0.97 Co 0.8 Mn 0.2 [Fe(CN) 6 ] 0.81 •2.2H 2 O. (G) GCD curves at 0.5 A/g of Mn-based PBA, Co-based PBA, and Co 0.8 Mn 0.2 PBA samples.The samples in (G) were synthesized without the addition of sodium citrate.

Fig. 4 .
Fig. 4. Comparison of structure and composition for cathode material at fully charged and discharged states.(A) XRD patterns of K 0.97 Co 0.8 Mn 0.2 [Fe(CN) 6 ] 0.81 •2.2H 2 O at pristine and fully charged/discharged states.(B and C) Na 1s and K 2p XPS spectra of K 0.97 Co 0.8 Mn 0.2 [Fe(CN) 6 ] 0.81 •2.2H 2 O at the first fully discharged/charged states.(D) High-resolution Na 1s and K 2p XPS spectra of K 0.97 Co 0.8 Mn 0.2 [Fe(CN) 6 ] 0.81 •2.2H 2 O at the fully discharged/charged states in the second cycle.The fully discharged state was obtained by discharging the cathode to 0.05 V, while the fully charged state corresponds to 1.1 V.

Fig. 5 .
Fig. 5. Electrochemical performance of anode material.(A) CV profiles at different scan rates, (B) GCD curves at different current densities, (C) rate performance, and (D) cycling stability at 1.1 A/g for NaTi 2 (PO 4 ) 3 /C.The inset in (A) shows the relationship between the peak currents and scan rates.(E) High-resolution Na 1s XPS spectra of NaTi 2 (PO 4 ) 3 /C in the fully discharged and charged states.

Fig. 6 .
Fig. 6.Electrochemical performance of full cell.(A) CV curves at different scan rates, (B) GCD profiles at different current densities, (C) rate performance, and (D) cycling stability at 5 A/g.(E) Ragone plots for the state-of-art rechargeable seawater batteries and aqueous Na + batteries.(F) An optical photograph of the assembled RSWB pouch cell for powering a light-emitting diode display screen.